- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Wow Monads!
by David McClain 17 Apr '17

17 Apr '17

That was absolutely excellent !! Great video presentation by Douglas Crockford. It took me about 2 hours to slowly advance through it, stopping along the way to translate his JS into Lisp to try out the Monads. This is by far the clearest explanation of Monads that I have seen, from among at least 2 dozen different videos and blogs. Even clearer than that Wadler paper. I highly recommend viewing this presentation. As for the rest of them, I hearken to the words of another great scientist who once stated that: “If you can’t explain a problem to your grandmother so that she understands it, then you don’t really understand the problem yourself…" - DM >> On Apr 16, 2017, at 15:43, MarLinn <monkleyon(a)gmail.com> wrote: >> >> On 2017-04-16 03:13, John Wiegley wrote: >>> MD> Anyone else have Lisp experience using Monads? Did it offer some major enhancements for you? >>> >>> In Lisp, I don't think the abstraction buys you much, because all the functionality you need (state, exceptions, runtime, etc) are always available to all expressions. They become an excellent way of representing "extensible composition" in a pure language that otherwise could not allow those things, but I don't see that you need it for an untyped language like Lisp. >> >> May I present to you the lecture "Monads and Gonads" by Douglas Crockford (https://www.youtube.com/watch?v=b0EF0VTs9Dc) He shows that monads can make even a language like JavaScript useful (not his words). And the talk is one of the better monad tutorials out there to boot, not least because it's different from all the others. >> >> My own conclusion from it is that I would maybe even turn your argument around and claim that especially a language that has as few tools to keep programs sensible as JS can benefit greatly from the structure a monad provides. The benefits will be different for Lisp, but I imagine there might be some nice use cases as well. After all, structures are a great tool even if you're not forced to use them. ;) >> >> On another note: the more I work with monads and their brethren, the more I find myself thinking in terms of (a -> f b) functions instead of things like bind. Not only is it closer to the mathematical basis, but there's also the close relationship to lenses. I mention this because my feeling is that this type of function is a more usable puzzle piece in languages with limited syntax support. Especially if you also implement functors. But that's just unsubstantiated gut feeling. >> >> Cheers, >> MarLinn >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >

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Ensuring that blank-state has been handled in UIs?
by Saurabh Nanda 17 Apr '17

17 Apr '17

Hi Everyone, In most user-interfaces we need special-treatment for blank states (zero-item state). For example: * In case there are no customers, don't show an empty table, instead show the message "You seem to have no customers, why not start by creating one..." * In case there are no bookings, don't show an empty table, instead show the message, "No bookings here. Want to create one?" * In case there are no search results, don't show an empty table, instead show the message, "No items found. Undo your last filter?" In some cases, the one-item case also needs special treatment, but I'm unable to come with relatable examples at the moment. A lot of times the dev simply forgets about the blank state, and it is caught during QA, which results in a quick-fix on the following lines: if length item > 0 then displayTable else showBlankState if Data.Map.Strict.size map > 0 then displayTable else showBlankState if Data.Set.Ordered.size set > 0 then displayTable else showBlankState Is there a way to prevent this bug from the get-go? Can we use the type-system (or anything else) to enforce the dev to at least _think_ about the blank state? Obviously, the type system can't help us with _what_ needs to be done with the blank state, but at least it can _remind_ the dev at compile-time about handling blank states. -- Saurabh.

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ANN: projects available for adoption
by Simon Michael 17 Apr '17

17 Apr '17

Hi all, to increase focus on other things, I am offering the following projects for adoption. Are you interested in one of these ? Would you like to pick it up and make it shine ? Ideally you are a regular user of the project yourself, and will be a more active maintainer than I have been. Or, you might like to try being co-maintainer or release manager for a time. Let's talk! http://joyful.com/shelltestrunner - compares command-line programs or shell commands with expected output. http://hackage.haskell.org/package/quickbench - produces quick, readable comparative benchmarks of program invocations. http://hackage.haskell.org/package/rss2irc - an IRC bot that announces any RSS/Atom feed to IRC. Formerly used for hackagebot in #haskell. http://joyful.com/fungen - the oldest and easiest way to build cross platform, easy to install 2D games in haskell. Also: http://planet.darcs.net <http://planet.darcs.net/> - Darcs-related feed aggregator (Planet Venus) http://planet.squeak.org <http://planet.squeak.org/> - Squeak Smalltalk-related feed aggregators (Planet Venus) http://hub.darcs.net/simon/darcsum <http://hub.darcs.net/simon/darcsum> - emacs UI for darcs (think mini-magit) (elisp) http://zwiki.org <http://zwiki.org/> - a powerful wiki engine built on the Zope 2 platform. (Long shot, just putting it out there.) (python) Best! -Simon

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Jane Street Question
by Michael Litchard 16 Apr '17

16 Apr '17

Could those of you who have worked at Jane Street comment on how often a hiring round occurs? Once a quarter? Twice a year?

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Re: [Haskell-cafe] Downsides of the Prompt Monad
by Oleg 15 Apr '17

15 Apr '17

David Menendez wrote > It's worth noting that Prompt p is the free monad for a type constructor p, > just as Free f is the free monad for a Functor f. As such, when p is a > Functor, they are isomorphic. > data Free f a = Var a | Wrap (f (Free f a)) > newtype Prompt p a = > Prompt { unPrompt :: forall b. (forall i. p i -> (i -> b) -> b) -> (a -> b) -> b } > -- equivalently, Prompt p a = Done a | forall i. Prompt (p i) (i -> Prompt > p a) Indeed. The Freer monad paper, Sec 2.4, describes this correspondence in general: type FFree p = Free (Lan p) (FFree p is the alternative Prompt p in David's message) where Lan p is a (simple version) of the left Kan extension that turns any p :: * -> * into a functor. Once (Lan p) is a Functor, the standard free monad construction applies. The Freer monad paper then goes on to say that since the continuation is made explicit, it can be represented in a better way than just a function.

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Haskell plugin dynamic linking issue
by Lana Black 14 Apr '17

14 Apr '17

Hello, I'm working on a Haskell plugin (a shared library) for a C program. When I load the plugin, I get the following error: > ./open dlopen() failed: /usr/lib64/ghc-8.0.2/ghc-prim-0.5.0.0/libHSghc-prim-0.5.0.0-ghc8.0.2.so: undefined symbol: stg_thawArrayzh open is just a testing wrapper around dlopen(). I found out that stg_thawArrayzh symbol is exported by the rts, and my library isn't linked to it, and neither is ghc-prim. How do I fix that? Adding -lHSrts-ghc8.0.2 to ld-options doesn't seem to have any effect. Thanks.

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ANN: Nomyx 1.0, the only game where you can change the rules
by Corentin Dupont 14 Apr '17

14 Apr '17

After a loooooong wait... I am finally proud to announce Nomyx <http://www.nomyx.net> V1.0! Nomyx is a unique game where you can change the rules of the game itself while playing it. The major novelty in this version is that you can play even without knowing how to program. There is a library of rules that are ready to be proposed. Let's start a new game! You can login here: http://www.nomyx.net:8000/Nomyx The games are discussed on the mailing list: nomyx-game(a)googlegroups.com Here is a video introduction of the game: http://vimeo.com/58265498 I created a tutorial to learn how to play: http:// corentindupont.info/blog/posts/Programming/2014-09-23-Nomyx-Language.html Some background: this is an implementation of a Nomic [1] game in Haskell (I believe the first complete implementation of a Nomic game on a computer). At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Cheers, Corentin [1] www.nomic.net

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looking for a file mentioned on haskell.org
by Michael Litchard 13 Apr '17

13 Apr '17

Can someone post/send a copy of the file p83.dat mentioned in the below link? https://wiki.haskell.org/99_questions/80_to_89

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RFC: Handling marginally bad data via logging "soft-errors"
by Saurabh Nanda 13 Apr '17

13 Apr '17

Cross-posted from https://www.reddit.com/r/haskell/comments/64ymup/rfc_handling_marginally_ba… Hi Everyone, I would like to hear some perspectives from people who have more experience in handling production code in Haskell. SCENARIO: You're loading data from a DB (or any other external source) and are expecting certain fields to be present. You end-up loading a record/row which has all the strictly necessary fields, but a not-so-necessary field is missing. Example: loading a full name, stored as the following HSTORE in PG: {'title' => '...', 'first_name' => '...', 'last_name' => '...'} For whatever legacy reasons, there is a possibility that some records may not have the title field, or may not have the last_name field. Now, the skies are not going to fall if you display the name with some components missing, so, you don't want to throw an error and terminate the entire operation. It's a "soft error" which you want to log, and proceed with a default value. OPTION 1: Use unsafePerformIO to avoid making an otherwise pure function, "impure", i.e. parseName :: HStore -> FullName -- uses unsafePerformIO to log soft-errors OPTION 2: Put the data transformation function in IO or the app-wide monad AppM because it will call the logging function, i.e. parseName :: HStore -> AppM FullName OPTION 3: Use a wrapper over Writer monad, called ErrorCollector, that forces the calling function to log while accessing the underlying value, i.e. parseName :: HStore -> ErrorCollector FullName logErrorAndExtract extraLoggingContext (parseName hstoreVal) THOUGHTS: -- The downside of Option 1 is that (a) it's unsafe, and (b) the log won't have surrounding context, for example it won't have the primary key of the row that was being loaded within which this error occurred. -- The downside of Option 2 is that it introduces AppM into the function signature, and even that log won't have enough surrounding context to help in debugging. -- The downside of Option 3 is that it pushes some logging responsibility to the calling functions, but gives the ability to put extra context when a soft-error occurs. How do others deal with this problem? -- Saurabh.

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ANN: HacFreiburg 2017, Friday 4 August - Sunday 6 August, Freiburg, Germany
by Stefan Wehr 12 Apr '17

12 Apr '17

Dear Haskellers, there is another Haskell Hackathon, taking place in Freiburg, Germany this year! When: Friday 4 August 2017 - Sunday 6 August 2017 Where: Faculty of Engineering, University of Freiburg, Germany (https://goo.gl/maps/ZcHWLpMPhtj) Info page: https://wiki.haskell.org/HacFreiburg2017 Organizers: Peter Thiemann, Alexander Thiemann, Stefan Wehr HacFreiburg is another Haskell Hackathon, where Haskell programmers from all around the world meet in Freiburg, Germany, to discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! The Haskell Hackathon in Freiburg is part of the "Sommercampus 2017" (https://sommercampus.fachschaft.tf/en/) organized by the Faculty of Engineering at the University of Freiburg. Hope to see you there, Stefan (with Peter and Alex)

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